On Mechanisms Controlling the Interannual Variability of the Summertime Bottom Low‐Oxygen Zones in the Southern Bohai Sea

In the southern Bohai Sea (BS), the formation and maintenance of the summertime bottom Dissolved Oxygen (DO) depletion regions are multifactor controlled. A coupled physical‐biogeochemical model was developed to simulate the DO dynamics of 2014 and 2019 in the southern BS. Bottom low‐oxygen zones were identified between the 20–25 m isobaths northwest of the Yellow River estuary in both years, with an additional zone observed in Laizhou Bay in 2019. Four sensitivity experiments were designed by exchanging the realistic Yellow River discharge and surface wind of the two years to investigate the dynamical mechanisms underlying the interannual variability of the low‐oxygen zones. The results indicate that the low‐oxygen zone along the 20–25 m isobaths was not significantly affected by variations in river discharge, whereas the low‐oxygen zone in Laizhou Bay was strongly influenced by discharge levels. Persistent southerly winds in 2019 drove the intrusion of Northern Yellow Sea bottom water along the 20–25 m isobaths, enhancing vertical stratification and promoting local DO depletion. In Laizhou Bay, the wind also caused a redistribution of the Yellow River discharge, which intensified stratification and led to the emergence of a low‐oxygen zone in 2019. Conversely, interruptions in the southerly summer monsoon, as in 2014, disrupted the development of low‐oxygen zones, limiting their spatial extents. Strong synoptic wind events or rapid changes in wind direction increased the vertical sheer mixing that dissipated low DO water in the shallow areas in 2014.